1. Field of the Invention
This invention relates generally to magnetic resonance imaging (MRI) which employs nuclear magnetic resonance (NMR) phenomena. The invention is particularly directed to method and apparatus for maintaining the requisite homogeneous NMR polarizing magnetic filed B.sub.o within an altered volumetric shape thus facilitating MRI within non-spheroidal image volumes. In effect, the invention utilizes adaptive main magnet shimming for MRI.
2. Description of Prior Art
MRI is by now a well known and commercially available non-invasive procedure for obtaining diagnostic information about the internal structure of living tissue. In very brief summary, NMR nuclei (e.g., a significant proportion of hydrogen nuclei in the human body) are nominally aligned with an intense superimposed static homogeneous magnetic field B.sub.o. By applying predetermined sequences of NMR RF nutation pulses and magnetic gradient pulses of selected duration and in selected directions (e.g., to selectively cause transient gradients in the B.sub.o magnetic field along the usual orthogonal X,Y,Z coordinate directions), some of these nuclei are disturbed in a predetermined manner from their quiescent orientations. As they tend to return to the quiescent orientation with B.sub.o, they emit characteristic spatially encoded RF signals which are detected, digitized and processed in known ways to produce a visual image representing the distribution of NMR nuclei (e.g., along the selected planar "slice" of volumes of living tissues within a predetermined image volume).
In conventional commercially available MRI systems, different geometries and coordinate systems are used to practice MRI. For example, one common arrangement uses a solenoidal cryogenic super-conducting electromagnet to produce the nominally static homogeneous quiescent magnetic field B.sub.o along a Z axis centered within the bore of the solenoid. Another arrangement uses an array of permanent magnets and magnetic circuit yokes between enlarged transverse pole pieces disposed above and below the image volume. In whole-body MRI systems, the dimensions of the main magnet and of the associated gradient coils (and some RF coils) utilize relatively large scale geometry for a number of reasons. First of all, the system must be large enough to accommodate a human body (or at least the portion of the human body that is to be imaged). But, perhaps even more importantly, relatively large geometries are utilized to obtain the required uniformity, linearity and/or reproducibility of magnetic fields within the image volume, the image volume itself comprising a relatively small and limited portion of the entire volume bounded by such structures.
While the actually imaged volume may depend upon a number of factors (e.g., the volumetric space to which the RF coils are coupled), it is necessarily limited in potential size to the volume in which the main magnet is capable of providing a substantially homogeneous NMR polarizing field B.sub.o. This is so because the ability of depends on the homogeneity of the static magnetic field B.sub.o MRI to provide a spatially accurate image of anatomy as well as the linearity of the magnetic gradient coils. Typical commercial MRI systems are designed so as to produce the requisite magnetic field homogeneity and gradient coil linearity over a generally spherical volume of approximately 40 cm diameter.
The linearity of the magnetic gradient coils can be improved substantially through a series of well known procedures for creating higher order corrections to the gradient coil field. Generally, however, linearities on the order of even 1% provide sufficient linearity for most human tissue imaging.
The requirement of homogeneity for the background or polarizing NMR field B.sub.o is a more exacting constraint. Typically, commercial MRI systems require homogeneity on the order of 20 to 50 parts per million throughout the potential imaging volume before possible inhomogeneity artifacts can be substantially disregarded (at least with current commercially available MRI processes). Using standard main magnet MRI technology and shimming techniques, the required homogeneity is achievable with a relatively modest effort (in many cases). However, improvements beyond the existing acceptable levels of homogeneity and/or maximum potential imaging volumes encounter significant difficulty. For example, if one wishes to increase the potential imaging volume (e.g., by increasing the diameter of the suitably shimmed spherical volume of homogeneity), there is soon encountered the very difficult problem of providing suitably increased magnet shimming regions.
Conventional main magnet shimming (which may include an electromagnet shim coil) so as to produce spherical volumes of substantially homogeneous polarizing field B.sub.o are realistic for most imaging situations involving heads, extremities or abdominal regions. However, for certain imaging procedures (most notably of the spine), it would be of considerable benefit to provide an ellipsoidally shaped volume of homogeneous NMR polarizing field. In this way, in conjunction with suitably dimensioned RF surface coils or the like, a greater length of the spine might be imaged in a single MRI data acquisition sequence.